I think it's more easy. When they subscribe to your blog, you should include the aspect that the data processor (handling the email) do some additional tracking in order to pursue a specific service. Regarding the consent (article 7 of the GDPR), provide a clear description why you do tracking and the objective. The transparency aspect is very important.

PS: It's usually better to have a look at the GDPR on the official website of EUR-Lex. https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CEL...